700B blew one Right channel 5A supply fuse

[laatsch55]

Takes more voltage on the input to turn things on, but you have a short again somewhere. Check your work again, and check for solder bridges, stuff like that...

 

[laatsch55]

With the amp off, check the shell of the RCA-410's to ground...

 

[62vauxhall]

I looked at the places on the back wall where I've disconnected and re-connected resistors and diodes and can see no solder bridges or crossed leads. I've been using a table mounted illuminated magnifier so not just unaided vision. However, I am really only afforded a top on view.

I pushed at connections and did find a problem but it wasn't the problem. The 180 Ohm resistor either R37 or R36, when I re-connected it earlier, I re-soldered it badly. But after fixing that, there was no change to bias voltage.

The DMM, to the best of my knowledge (which is obviously not much), did not detect continuity anywhere it shouldn't have been. But, I could have overlooked opportunities.

The only thing I can think to do now but it's too late for me to make noise tonight, is use my air compressor to blast out any stray bits of solder that may be present.

Is it possible one of the back wall resistors shorted, or a diode? Got too hot maybe from soldering or old age?

Don't know if you would consider it too extreme, but what about clearing the deck, so to speak, of the Left channel resistors and diodes on the back wall and replacing all of them? I thought that would be one way of finding any solder bridges or blobs.

Since I have not done anything to the board other than connect the wires, and they look OK, I assume that the problem is not there but somewhere on the back wall.

I unsoldered and re-soldered around the Normal/Direct switch and copper plate but again, could not see an obvious short. I checked for a short in those new co-axial leads I put in and they were OK too.

 

[62vauxhall]

With the amp off, check the shell of the RCA-410's to ground...

Get zero - to bus bar or chassis.

 

[laatsch55]

damn Gary, are the power supply voltages symmetrical?? On the rails?

 

[laatsch55]

You can check each resistor indivudually and the diodes on the back wall without pulling themMake a quick check of the diodes and tranny's on the board too, don't rule anything out. Also check the variabl;e resistor for the bias adjust with the meter and see if they vary the resistance...

 

[62vauxhall]

damn Gary, are the power supply voltages symmetrical?? On the rails?

With variac still at 29 volts.

The left channel rail pairs, depending on which side the leads are attached:

plus or minus 8.25 volts
plus or minus 1.124 volts

Right channel rail pairs:

plus or minus 4.59 volts
plus or minus 1.50 volts

 

[laatsch55]

Not the transostors the power supply rails....check at the power supply cap terminals...

 

[62vauxhall]

You can check each resistor indivudually and the diodes on the back wall without pulling themMake a quick check of the diodes and tranny's on the board too, don't rule anything out. Also check the variabl;e resistor for the bias adjust with the meter and see if they vary the resistance...

Just checked the variable resistor and it goes from 11 Ohms to 5.43K Ohms.

To recap;

Check all transistors on the board.
I can probably do that tonight.

Check all diodes and resistors on the back wall and the board.
Disconnecting a leg of each to test them will take a long time. Would not be able to finish that until sometime tomorrow - probably late.

I have a 9AM appointment in Vancouver to get the valves adjusted on my motorcycle which is a 3 hour job so not home till around 1PM. At 6PM I have to attend something and gone until 9PM.

I will start unsoldering and checking diodes and resistors when I get back from Vancouver tomorrow but have no idea what time of day, or rather night, I'll be finished.

I am certainly not looking forward to altering that nice pristine board. I remember what the last one looked like afterwards.

 

[62vauxhall]

Not the transostors the power supply rails....check at the power supply cap terminals...

OK. Black lead to bus bar and Red lead to caps positive.

Plus 5 volts and minus 5 volts respectively.

 

[laatsch55]

With the amp on? No need to desolder all those on the back wall, they can be checked in circuit...w're not going to alter that board, just a quick BE,BC check on all tranny's and a forward/reverse check on the diodes...no desoldeing on any thing......yet.....

 

[NavLinear]

Get zero - to bus bar or chassis.

Is this a resistance measurement from the RCA 410 collector to chassis/ground?

 

[62vauxhall]

Is this a resistance measurement from the RCA 410 collector to chassis/ground?

Yes. Thought I was supposed to check for shorts.

 

[62vauxhall]

With the amp on? No need to desolder all those on the back wall, they can be checked in circuit...w're not going to alter that board, just a quick BE,BC check on all tranny's and a forward/reverse check on the diodes...no desoldeing on any thing......yet.....

Yes. Amp was on. Same 29 volts on variac. Good to learn no unsoldering - yet.

Will post later on BE, BC transistor check and fwd/rev diode check.

Amp will be off to do this.

 

[NavLinear]

Yes. Thought I was supposed to check for shorts.

The collector should be isolated from ground by the mica isolator / Sil-Pad.

 

[laatsch55]

Yep, that's what i wanted to see Gary..

 

[laatsch55]

Yes. Amp was on. Same 29 volts on variac. Good to learn no unsoldering - yet.

Will post later on BE, BC transistor check and fwd/rev diode check.

Amp will be off to do this.

29 volts on the variac and all you had was +5/-5 on the rails??? That's through the DBT too, so that makes a differen=ce....5 volts is not enough too turn anything on.( ok Joe, most of it).. Monitor rail voltage till it gets to 40 volts, if you can get it without the bulb looking like the sun...

 

[62vauxhall]

The collector should be isolated from ground by the mica isolator / Sil-Pad.

Good point, did not think of that.

 

[62vauxhall]

29 volts on the variac and all you had was +5/-5 on the rails??? That's through the DBT too, so that makes a differen=ce....5 volts is not enough too turn anything on.( ok Joe, most of it).. Monitor rail voltage till it gets to 40 volts, if you can get it without the bulb looking like the sun...

Brought the variac up to about 117 volts. DBT was bright but not as bright as a 100 watt bulb can be. Voltage measured between bus bar and storage cap positive was not even 10 volts.

Any chance this is a transformer problem?

FWIW, also noticed the panel lights were not on at all. Before replacing those bias transistors and .33 Ohm resistors, the did come on but faintly.

 

[laatsch55]

Check fuses gary...